In vertebrates, the signalling pathways which determine retinal cell lineage have not been well defined. Retinal progenitors begin as multipotent cells which undergo a systemic restriction in cell fate. Specification of retinal fate involves multiple step: mesoderm induction, neural induction, regionalization of CNS, optic vesicle induction, and specific retinal cell phenotype induction. The main objective of this research is to elucidate some of the mechanisms by which induction of and commitment to retinal cell fates is attained during development. Since both cell position and competence are critical determinants of cell fate, this proposed research specifically examines both potential signals (positional information) and the signal transduction machinery (competence) using the frog, Xenopus laevis, as a model system. The expression of functional and mutant receptor molecules and signalling factors will be genetically manipulated to define which signalling events are necessary and/or sufficient to determine retinal cell fate. Using exist detailed fate maps and lineage markers, specific mRNAs encoding components of putative retinal signalling pathways will be precisely targeted to retinal precursor cells. Candidate signal pathways will be eliminated in retinal precursors and the effects of ectopic expression in non-retinal precursors will also be tested. While these experiments will not unravel the entire complexity of the cell signalling required for the initial steps of retinal fate determination, they will provide the first molecular description of the required pathways.